The present invention relates to improved resistor compositions and resistors produced therefrom. More particularly, the present invention relates to ruthenium oxide-glass type thick film resistor compositions and resistors produced therefrom in which resistor properties such as temperature coefficient of resistance (hereinafter referred to as TCR), noise and voltage coefficient of resistance (hereinafter referred to as VCR) are surprisingly improved in high resistance ranges.
Thick film resistors produced by firing a film comprising ruthenium oxide and glass onto a surface of an electrically insulating substrate change in resistance over a wide range of from a few ohms per square to 10 megohms per square the weight ratio of ruthenium oxide to glass varies in the range of from 60:40 to 5:95. Thus, usually desired resistances are obtained by controlling the weight ratio.
In practical use of such resistors, it is preferred that their resistances not change due to changes in ambient temparature, in other words, that the TCR is zero. As to the relation between resistance and TCR, although there are differences depending on particle size of ruthenium oxide, glass composition and particle size of glass, generally speaking, a resistance region having a TCR of zero exists only in medium resistance ranges. However, when the resistance is below the region of zero TCR, TCR values increase positively with decreasing resistance values, and, on the other hand, higher resistance ranges above the region render TCR values more negative with increasing resistance values. Thus, the conventional resistors described above are liable to be affected by ambient temperatures in almost all resistance ranges and it is very difficult to control the absolute TCR to zero.
As conventional means for improving TCR, it has been well-known to add various additives to the resistor compositions. For example, TCR adjusting additives such as MnO.sub.2, Al.sub.2 O.sub.3, TiO.sub.2 and ZrO.sub.2 have an effect of shifting TCR in a negative direction, and, thus, these additives are effective and useful for resistors in low resistance ranges having a highly positive TCR. However, as to the cases of resistors in high resistance ranges, no satisfactory solution has been found to date despite many studies and attempts. For example, in U.S. Pat. No. 3,324,049, addition of copper oxide to resistor compositions and the use of glass containing copper oxide as a glass forming constituent are disclosed as means for adjusting TCR.
The addition of copper oxide or the use of copper oxide containing glass can allow highly negative TCR values to come near to zero, but, simultaneously, unfavorable reduction in resistance and deterioration of VCR are unavoidably caused. As a way of further adjusting TCR values, British Pat. No. 1,470,497 describes that addition of colloidal AlOOH to resistor compositions serves to adjust the TCR in a positive direction. However, this adjusting method simultaneously causes a significant reduction in resistance. As a result, it is impossible to obtain resistors having TCR values close to zero in the high resistance ranges. Further, TCR has been adjusted by coarsening respective particle sizes of ruthenium oxide and glass. However, this method results in an unfavorable increase of noise level and a wide variation in resistance value and, thus, this method is not practicable. As mentioned above the conventional methods cannot satisfactorily improve the TCR property in the high resistance ranges.